when you order bmw oil in the u.s. it comes in quarts
the bmw manual says to put in liters which have 6% more oil in them than quarts. if you want to make up the difference with what you got layin' around. i'd do it, which i already do (if i bother to do it at all).
i change the oil in the fall and halfway through the summer pretty much regardless of mileage. the filter only gets changed in the fall.
maybe if you use oil from fake dinosaurs, you might not wanna mix it with oil from real ones, although i doubt the effect on the engine could be quantified by a scientific scientist.
oh yeah, i got my first bmw in 1964. (i'm a lot younger than i look).